1. Field of the Invention
The invention relates to a manufacturing method for a sintered compact, in which magnetic powder for rare earth magnet is formed by hot pressing, thereby manufacturing a sintered compact that is a precursor of rare earth magnet.
2. Description of Related Art
Rare earth magnet that uses rare earth elements such as lanthanoid is also referred to as permanent magnet, and is used for motors that structure a hard disk and MRI, and drive motors for a hybrid vehicle, an electric vehicle, and so on.
As an index of magnetic performance of the rare earth magnet, there are residual magnetization (residual magnetic flux density) and coercivity. However, for an increase in an amount of heat generation due to size reduction and higher current density of motors, demands are growing further for heat resistance of rare earth magnet to be used. Therefore, how to maintain magnetic characteristics of magnet when used at high temperature is one of important study subjects in the art.
Rare earth magnet includes generic sintered magnet in which a scale of a crystal particle that structures the structure (main phase) is about 3˜5 μm, and also nanocrystal magnet in which a crystal particle is miniaturized to a nanoscale of about 50 n˜300 nm. Among them, nanocrystal magnet has now attracted attention, as nanocrystal magnet is able to reduce an addition amount of expensive heavy rare earth elements or eliminate addition of heavy rare earth elements while achieving miniaturization of the above-mentioned crystal particles.
An example of a manufacturing method for rare earth magnet is outlined. A method for manufacturing rare earth magnet (oriented magnet) is generally used, in which a quenched thin belt (a quenched ribbon), which is obtained by rapidly solidifying, for example, Nd—Fe—B-based molten metal, is fabricated, and magnetic powder fabricated by crushing the quenched thin belt is made into a sintered compact while being formed by hot pressing. Then, plastic working is performed on the sintered compact in order to give magnetic anisotropy.
When fabricating a sintered compact by hot press forming of the foregoing magnetic powder, if a mass of the magnetic powder housed in a forming mold is heated from outside to densify the magnetic powder in a short period of time, there will be a large temperature difference between an inner region and an outer region of the mass of the magnetic powder, and temperature of the outer region becomes higher than that of the inner region. Then, at a point in time when temperature of the inner region reaches temperature required for the densification, the outer region has already been exposed to an atmosphere at coarse crystal particle generation temperature or higher for a long period of time.
In a case where the magnetic powder is nanosized powder, deterioration of magnetic characteristics is unavoidable because finally obtained nanocrystal magnet contains coarse crystal particles.
In Japanese Patent Application Publication No. 2003-342618 (JP 2003-342618 A), a manufacturing method for anisotropic earth magnet powder is disclosed. In this method, preliminary heating is performed, in which a metal cylinder filled with super-quenched powder is held in an atmosphere at temperature lower than crystallization temperature of a magnet alloy, thereby allowing temperature of the super-quenched powder to reach temperature close to the atmosphere temperature. Then, the temperature is increased to about 650 to 900° C. and uniaxial compression is performed. Thus, it is possible to obtain magnet powder while preventing coarsening of particles. To be more specific, magnetic powder that is preliminarily heated in a muffle furnace is moved to a heating press and pressed.
As stated above, after preliminary heating of magnetic powder, the magnetic powder is moved to a forming mold (a heating press) for main heating. Therefore, it is not possible to avoid a problem that temperature of the magnetic powder, which is preliminarily heated to a desired temperature, is decreased. Then, when the magnetic powder is preliminarily heated to higher temperature to allow a temperature decrease of the magnetic powder, then coarsening of crystal particles could happen.